1. Field of the invention
The present invention relates to a method of restraining a diffusion of tritium and an apparatus for same.
2. Description of the Related art
In general, a fast breeder reactor comprises an arrangement of devices as shown in FIG. 3. In FIG. 3, a reactor vessel made of stainless steel is indicated at 1. The reactor vessel 1 contains a reactor core 2 in which a nuclear fuel assembly comprising a fissionable material is placed. The reactor core 2 receives a control rod 3 including boron so that the control rod 3 can be pulled up and inserted into the reactor core 2 from above. The fast breeder reactor has a primary cooling system piping 4 made of stainless steel and causing liquid sodium as a coolant to pass through the reactor core 2 in the reactor vessel 1 and to circulate by means of a pump 5 between the reactor vessel 1 and an intermediate heat exchanger 6. The fast breeder reactor also has a secondary cooling system piping 7 made of stainless steel and causes liquid sodium as a coolant of a secondary cooling system to circulate by means of a pump 8 between the intermediate heat exchanger 6 and a steam generator 9. A steam system piping feeding water to the steam generator 9 to receive steam therefrom is indicated at 10.
Both the primary and secondary cooling system pipings 4 and 7 have heat reserving means. The arrangement of each of the heat reserving means is as follows: FIG. 4 illustrates the arrangement of a piping 11 which is assumed to represent each of the primary and secondary cooling system pipings 4 and 7. As shown in FIG. 4, a stainless steel strip 13 surrounds both the piping 11 made of stainless steel in view of both resistance against corrosion caused by liquid sodium and a high-temperature strength and a preheater 12 arranged in parallel to the piping 11, a heat reserving material 14 surrounds the stainless steel strip 13, and a thin steel strip 15 surrounds the heat reserving material 14. The thickness of each of the stainless steel strip 13 and the steel strip 15 is normally about 0.1 mm-0.2 mm. The reserving material 14 normally is a mixture including calcium oxide or silicon oxide as a main component.
Turning back to FIG. 3, a fission reaction in the reactor or a nuclear transformation caused by neutron irradiation of the boron used as a moderator of the control rod 3 produces tritium. For example, in a nuclear reactor of one million KW tritium of about 1 g/year occurs. The tritium is mixed with liquid sodium passing through the reactor core 2 and transferred through the primary cooling system piping 4 to the intermediate heat exchanger 6. Then, the tritium permeates the wall of a heat-transfer pipe of the intermediate heat exchanger 6 and enters liquid sodium contained in the secondary cooling system piping 7 to reach the steam generator 9. Thus, the tritium permeates the pipings and devices, so that the tritium tends to readily permeate and diffuse in atmospheres outside of the devices. In particular, it is well known that tritium permeates stainless steel used to constitute the devices of a nuclear power plant. Thus, tritium permeating e.g. the piping 11 inside to outside permeates the heat reserving means to diffuse to the outside thereof.
Since as described above, the tritium which has once diffused continues to diffuse and in the worst case, might diffuse in the atmosphere, the diffusion of the tritium must be restrained.